Stability of 3CaO · Al2O3 · 6H2O in KOH + K2CO3 + H2O system for chromate production

In the novel clean chromate production process, the alkali liquor recycled to decompose the chromite ore mainly consists of KOH and K2CO3 which accumulates aluminium impurity and affects the quality of the product greatly. Aluminium impurity can be removed by adding CaO to precipitate as 3CaO · Al2O3 · 6H2O (C3AH6). A study of the effects of various parameters, such as KOH concentration, K2CO3 concentration and temperature on the behaviour of C3AH6 show that C3AH6 is decomposed to 3CaO · Al2O3 · CaCO3 · 11H2O and CaCO3 below 150 g L− 1 KOH, and decomposed to Ca(OH)2 above 150 g L− 1 KOH. With K2CO3 concentration increasing, C3AH6 decomposes significantly, which results in more CaCO3 or 3CaO · Al2O3 · CaCO3 · 11H2O produced. Temperature has a large positive effect on the decomposition of C3AH6 at 45 g L− 1 KOH but has no significant effect at 150 g L− 1 KOH. The optimal condition for removing aluminium impurity in the KOH + K2CO3 + H2O system is 150 g L− 1 KOH, 50 g L− 1 K2CO3 and 80 °C.